That's interesting. So the ISP is directing packets at the various customer routers using some other identifier than the WAN IP the router sees? MAC address or something?
HFC (Hybrid Fiber Coaxial) and FttN (Fiber to the Node) are such examples. The rest of them if you don't have a static IP or an internet routable address, use NAT (Network Address Translation) and in some even more exotic environments they use Port Address Translating (PAT), but it's been many years since I've seen PAT in any way at commercial, residential or enterprise setups in ISP space.
But PAT is quite common in the reverse and forward proxy space.